This invention relates to a power fail monitor and transfer circuit for use in a private branch exchange, to provide means for placing or receiving calls outside the private branch exchange during a failure, such as a commercial power failure, an internal component or software failure, or an internal DC supply failure.
It is common practice to provide means at a private branch exchange whereby, in the event of a failure of the above-described type, one or more pre-designated stations of the private branch exchange will be cut through directly to the central office where an auxiliary power supply is available. The means, generally a power fail transfer scheme or arrangement, normally used have at least one drawback which is far less than satisfactory. For example, with most of these schemes, during power restoration, any conversation on the established power fail loop is cut off. More particularly, most present power fail transfer schemes provide at least one and usually more metallic transmission loops from the subscribers on the private branch exchange to a dedicated trunk line or lines to the central office. The central office battery is then utilized as the transmission media and on incoming calls the central office extends ringing current directly to the private branch exchange subscriber. When power is restored, these metallic transmission loops are automatically opened, so that any conversation on a transmission loop is cut off. An arrangement having this drawback is disclosed in, for example, U.S. Pat. No. 3,506,789.
Accordingly, it is an object of the present invention to provide an improved power fail monitor and transfer circuit.
A more specific object is to provide such a power fail monitor and transfer circuit wherein existing telephone conversations are not interrupted during power restoral.
A feature of the power fail monitor and transfer circuit is that the trunk circuit can be automatically busied out so that, upon power being restored, the trunk circuit is marked busy to the central processor to thus prevent the central processor from attempting to seize the trunk circuit.